Chemistry - A Molecular Science

Chapter 9 Reaction Energetics

b) H

COH(g) + HF(g) 3

→

H

CF(g) + HOH(g) 3

First, draw the Lewis structures. Examination of

the Lewis structures reveals that neither

the C-H bonds nor the O-H bond are affected by the reaction, so only the bonds shown as dotted lines are broken or formed.

H

H

CH H

O

HF

HC

H H

F

++

H O

(^) H
Breaking the C-O and H-F bonds and forming the C-F and O-H bonds yields the following:
ΔH ~ D(C-O) + D(H-F) – D(C-F) – D(O-H) = 358 + 565 – 485 – 463 = -25 kJ
Breaking the three C-H bonds and the O-H bond and then reforming them would yield the same answer because the energy required to
break them is identical to that released
when they reform.
9.5
ENTROPY
A spontaneous process is one that, once star
ted, takes place without intervention. For
example, once a ball begins to roll down a hill,
it does so spontaneously, but it can sit at
the top of the hill and not roll down without some kind of intervention to get it started. However, it rolls uphill only with intervention ev
en after the climb is started. As another
example, gasoline reacts spontaneously with oxy
gen, yet gasoline can be stored in air
because it needs intervention (a spark or flame)
to get started. However, once the reaction
is started, it continues spontaneously. As we shall see in Section 9.9, the initial intervention that is required to start a spontan
eous reaction is called the activation energy
for the reaction.
In this section, we show how to predict whether a process is spontaneous. Our first
hypothesis might be that a process is spontaneous if it is exothermic. After all, we have made frequent use of the concept that syst
ems strive to lower their potential energy.
However, recall from Chapter 7 that the potentia
l energy of the molecules in the gas phase
is much greater than that in the liquid ph
ase, yet liquids do evaporate spontaneously, so
evaporation is a process that proceeds spontane
ously to a higher potential energy. Clearly,
there must be another factor, and the other
factor is related to the manner in which
molecules distribute their thermal energy.
Recall from Section 7.2 that thermal energy (RT) is a measure of the average kinetic
energy (energy of motion) of the molecules in a system. Thus, we begin our treatment of
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